This invention relates to an apparatus for filtering gases and a method of operating an apparatus for filtering gases. More specifically the present invention relates to an apparatus and a method for filtering gases containing solid contaminants and for efficiently removing accumulated solids from the filtering elements in a safe manner.
In various modern applications, the efficiency of filtering devices is required to be extremely high, however the pressure drop over the filtering devices must not exceed a certain predetermined level. Thus there is an increasing need for more and more effective cleaning systems for the filters. Also durable high temperature systems are required for some applications. One such application is a Pressurized Fluidized Bed combustor connected to gas compressor/turbine system. In order to be able to use a solid fuel, such as coal or peat, for generating pressurized hot flue gases to be expanded in a gas turbine, one must have appropriate means to prevent the carry over to the turbine of particles from combustion of such solid fuels. An efficient system for filtering the gases would permit the use of such fuels.
Removal of the particulates from combustor exhaust gas effluent streams leading to a turbine can prolong the life of turbine blades and thus increase the reliability and efficiency of the whole process involved. Also, in some cases, governmental regulations relating to emissions permitted provide further impetus for commercialization of a system for filtering contaminated gases more efficiently.
It is known in the prior art to use various kinds of filters, such as bag house filters or ceramic filters for hot environments, in order to remove particulates from a gas stream. It is also essential to have the filter cleaned, e.g. after certain pre-determined operating periods in order to maintain the desired cleaning efficiency and pressure reduction of the gas over the filter. Commonly used methods of cleaning the filter employ a reverse directional pulse of gas for flushing the filter. This kind of method is disclosed e.g. in U.S Pat. No. 4,764,190. The filter elements are cleaned by use of a back pulse manifold that directs a pulse of gas flow into the filter elements in a direction opposite to normal direction of gas flow to be filtered, thus dislodging the accumulated cake (solids previously separated from the gas).
Also, in European patent 0 057 251 there is disclosed a filter having plurality of filter elements supported by a tube sheet, each filter element having an open top and disposed in apertures of the tube sheet extending into a lower chamber of the filter. There is also disclosed a system for back flushing the filter elements. The filter elements are cleaned by a pulse of high pressure gas from a gas delivery pipe; with the help of a venturi the gas pulse pulls additional gas from the immediate vicinity and increases the total amount of the cleaning gas, improving the cleaning effect. Further it is stated, that the system can include features which alleviate any undesirable interaction between the dirty gas entering the vessel and the downwardly flowing purged particles. According to the reference the inlets for gas to be cleaned are positioned high in the vessel, to promote a generally downwardly flow. EP 0 057251 discloses also a baffle having perforations in an upper section thereof, which baffle is disposed to form an annular space between a wall of the vessel, and the baffle functions to equalize the inlet flow of the gas to be distributed evenly to the filter elements, thus the gas flow is arranged to be generally downwardly near the inner surface of the baffle. The baffle extends from the bottom of the vessel to the upper section thereof.
EP-BI-0 078 678 discloses dislodging the partuculates from a filtering fabric by using an induced gas flow on the upstream (outer) side of the filter element. The gas is forced to circulate upwards along the outer surface of the filter fabric. Although the method may be feasible to some extent, the suggested inducement of the gas surrounding the filter element may introduce further difficulties by not allowing the dislodged particles to fall down.
Back pulse flushing is commonly practised so that only one or a few filters are cleaned at the same time while the others are in a normal usage mode, in order to ensure proper operation of the process while still maintaining the pressure reduction within a permitted range. When activating the back pulse system for cleaning a filter during a cleaning mode, the gas pulse is introduced into the filter causing the cake of particulate material accumulated on the surface of the filter to dislodge and to be entrained in a surrounding gas space by the force of the back pulse. However, since there are filters in normal usage in the vicinity of the pulsed filter, the dislodged particulates may be strongly entrained by those gases and carried into another filter. Thus the efficiency of the whole filtering device deteriorates. This causes a number of further difficulties, e.g. the sequence of flushing the filters must be multiplied causing a decrease in efficient filtering area available for filtering, and the density of particulates in the gas is generally increased in a filtering plenum, causing unnecessary abrasion of surfaces (which can lead to still further problems if cooled constructions are used in the filter assembly).
According to the invention, a method and apparatus are provided which substantially prevent particles which have been filtered out of a gas stream and collected on a filter from being entrained in a gas stream passing into association with another filter, as often happens in the prior art. Instead, the particles dislodged from a filter by a pulse of cleaning gas are directed to the particles outlet of the associated vessel containing the filters. By doing this, the invention achieves enhanced efficiency compared to the prior art.
According to one aspect of the present invention, a gas filtering apparatus is provided comprising the following elements: A hollow vessel having an inlet for gas with entrained particles to be filtered, an outlet for filtered gas, and an outlet for particles filtered from the gas, a first plenum for gas to be filtered, and a second plenum for filtered gas. At least one filter element disposed in the vessel between the first and second plenums, and containing at least one filter for filtering particles from gas. Means for introducing a pulse of cleaning gas to the filter for dislodging particles filtered by the filter which have collected on the filter, so that the particles may pass through the particle outlet of the vessel. And, shroud means associated with the filter element for preventing re-entrainment of particles dislodged by the pulse of cleaning gas in a stream of gas to be filtered.
Preferably a plurality of different filter elements, each having a plurality of filters associated therewith, are provided. The filter elements may be tubular or parallelepiped ducts which mount generally tubular (e.g. tubular or parallelepiped) filters (such as ceramic candle filter tubes, monolithic ceramic filers, fabric fibers in tubular configuration, etc.) so that the outer surface of the duct, surrounded by the shroud means, is substantially smooth, so that dislodged particles readily flow downwardly along it. The shroud means may comprise shroud walls in a honeycomb configuration, or individual parallelepiped or tubular shrouds substantially concentric with associated ducts, or zig-zag partition walls. The shroud walls may have openings therein which allow the passage of gas, but prevent the passage of dislodged particles.
Individual cleaning gas pulsing jets may be associated with the filters, and the associated shrouds may take the form of structures having an impingement portion at least as wide as the effective surface of the filter from which particles will be dislodged, and a material collecting portion.
In order to maintain the gas velocity within a desired range the gap between the shroud and the filter elements is designed to provide a desired gas velocity. Further, it is advantageous to dispose the cluster of filter elements so that the cluster comprises of a duct supported by a supporting plate, the supporting duct also functioning as a fluid communication path for gases, and a plurality of filter elements attached to the duct preferably in a manner which provides an annular volume between the shroud and the supporting duct which facilitates downward movement of the dislodged solids. This is preferably accomplished by mounting the filters to the duct so that they are substantially enclosed by the duct, with the filters attached to the duct in the vicinity of the portion of each filter which receives the gas to be purified. Thus the falling solids are allowed to slide smoothly along the surfaces of the shroud and the duct, not accumulating on the surfaces of the filters.
The shroud may be made of various materials, depending on the requirements of the environment. When filtering gases of high temperature, e.g. 1200.degree. C., a high temperature resistant material, such as high temperature metal or a ceramic material, is required.
It may be advantageous in some applications, e.g. when the wall of the shroud is solid, to have the diameter of the shroud vary in order to equalize the gas velocity in the volume between the filter and the shroud, one alternative is to form the shroud in conical shape, so that the diameter increases from the midpoint towards the upper and lower ends of the shroud, i.e. to provide a generally hourglass shape to the shroud. This is desired when the gases enter the volume between the shroud means and the filter through both ends, which is preferred since it provides even distribution of the gases over the filter area. Alternatively, the shroud may be provided with openings, formed so that the downwardly flowing dislodged particles are not allowed to exit the shroud.
Since the solids are easily entrained by the gases in the vessel, it is preferred to have at least one of the shrouds extend farther on its lower end than the others. That particular feature ensures that the upper level of a cloud of solids in the lower section of the first plenum is low enough not be influenced by the gas flow to the filters during normal operation.
According to another aspect of the present invention, apparatus for filtering gas is proved which comprises the following elements: A hollow vessel having an inlet for gas with entrained particles to be filtered, an outlet fro filtered gas, and an outlet for particles filtered from the gas, a first plenum for gas to be filtered, and a second plenum for filtered gas. A plurality of filter elements, each comprising a hollow duct, and each hollow duct mounting a plurality of individual filters therein, each individual filter for filtering particles from gas with entrained particles introduced into the gas inlet to the vessel. Means for introducing a cleaning gas pulse to the filters to dislodge particles separated from the gas with entrained particles which collect on the filters. And, a shroud surrounding portions of each of the hollow ducts adjacent the areas thereof from which particles will be dislodged for containing the dislodged particles and substantially preventing them from being entrained in gas, but rather directing the dislodged particles toward the vessel particles outlet.
According to yet another aspect of the present invention, a method of filtering gas is provided. The method comprises the following steps: (a) Introducing gas with entrained particles to be filtered into the vessel into operative association with the filters to filter particles from the gas, and discharging cleaned gas from the vessel. (b) At predetermined intervals, directing a pulse of cleaning gas toward at least some of the plurality of filters to dislodge filtered particles which have collected thereon. (c) Preventing the particles dislodged from the filters from being entrained in gas passing into association with other filters. And, (d) directing the dislodged particles so that they pass toward the particles discharge outlet. Step (b) is preferably practiced by directing a pulse of cleaning gas at only some of the filters, while other filters continue to operate to remove particles from gas passed therethrough, and may be practiced intermittently or periodically, for each of the filters. Step (c) is preferably practiced by providing a shroud surrounding the portions of each filter from which particles will be dislodged. Step (a) is preferably practiced to pass gas with entrained particles through openings in the shrouds, the openings being disposed so that dislodged particles will not pass therethrough, and is further practiced to pass gas with entrained particles at a pressure of between about 2-40 bar and at a temperature of between about 500.degree.-1200.degree. C. through the filters.
It is the primary object of the present invention to provide a method and apparatus, for filtering particles from gas, having enhanced efficiency by substantially preventing particles dislodged during cleaning of filters from being entrained in gas so that they have to be filtered out of the gas again. This and other objects of the invention will become clear from an inspection of the detailed description of the drawings, and from the appended claims.